1. Field of the Invention
The present invention relates to a variable gain amplifier used for cable broadcasting, radio communications, a magnetic recorder and so on, particularly to a variable gain amplifier provided with a dc offset canceling facility.
2. Description of the Related Art
Generally, it cannot be avoided in many amplifiers that the dc offset that is error components of a bias voltage and a bias current occurs in an output stage. So far various measures have been taken in order to remove this dc offset. There is, for example, an offset canceling system for removing the offset using a feedback amplifier circuit. In this case, the low frequency domain of the frequency band of a signal amplified by a predetermined gain (referring to as a lower limit frequency) fluctuates according to a change of the gain of the main amplifier. The frequency band of the amplified signal is narrower as this predetermined gain increases. Therefore, the low frequency domain of the signal that should be amplified at a high gain is not amplified, resulting in deteriorating quality of the signal.
It is an object of the present invention to provide a variable gain amplifier that can suppress fluctuation of a lower limit frequency according to change of the gain of a main amplifier circuit, and realize a good offset canceling.